Cathode of lithium battery and method for fabricating the same

ABSTRACT

A cathode of a lithium battery includes a composite film. The composite film includes a carbon nanotube film structure and a plurality of active material particles dispersed therein. A method for fabricating the cathode of the lithium battery includes the steps of (a) providing an array of carbon nanotubes; (b) pulling out, by using a tool, at least two carbon nanotube films from the array of carbon nanotubes to form a carbon nanotube film structure; and (c) providing a plurality of active material particles, dispersing the active material particles in the carbon nanotube structure to form a composite film, and thereby, achieving the cathode of the lithium battery.

RELATED APPLICATIONS

This application is related to commonly-assigned application Ser. No.12/080,714, entitled, “ANODE OF A LITHIUM BATTERY AND METHOD FORFABRICATING THE SAME”, filed on Apr. 4, 2008. Disclosure of theabove-identified application is incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to cathodes of lithium batteries andmethods for fabricating the same and, particularly, to acarbon-nanotube-based cathode of a lithium battery and a method forfabricating the same.

2. Discussion of Related Art

In recent years, lithium batteries have received a great deal ofattention.

Lithium batteries are used in various portable devices, such as notebookPCs, mobile phones, and digital cameras because of their small weight,high discharge voltage, long cyclic life, and high energy densitycompared with conventional lead storage batteries, nickel-cadmiumbatteries, nickel-hydrogen batteries, and nickel-zinc batteries.

A cathode of a lithium battery should have such properties as highenergy density; high open-circuit voltage versus metallic lithiumelectrode; high capacity retention; good performance in commonelectrolytes; high density; good stability during charge and dischargeprocesses, and low cost. Among various active materials, transitionmetal oxides and mixed transition metal oxides have received muchattention owing to their relatively high charge/discharge capacities inthe lithium batteries. At present, the most widely used cathode activematerials are spinel type lithium manganese oxide (e.g. LiMn₂O₄),olivine type lithium iron phosphate (e.g. LiFePO₄), and layered typelithium cobalt oxide (e.g. LiCoO₂).

However, the low conductivity of the active materials generally inducesa relatively large resistance in the cathode. As such, thecharge/discharge depth of the lithium battery is relatively low. Todecrease the resistance of the cathode, a conducting additive iscommonly mixed with the active material. The weight of the conductingadditive can reach to about 15%˜30% of the total weight of the cathode.If the conducting additive is increased and the weight of the batterymust remain the same, the amount of active material in the cathode mustbe reduced, and thus, the energy density of the lithium battery willsuffer.

To solve the above-described problem, carbon nanotubes as a novelconducting additive has been tested in cathodes of lithium batteries totake advantage of the excellent conductive properties thereof. In priorart, carbon nanotube powder is mixed with the active material byultrasonically agitating. Unfortunately, carbon nanotubes are prone toaggregation due to the extremely large specific surface area thereof,and as such, aggregated carbon nanotubes will not improve theconductivity of the cathode.

What is needed, therefore, is to provide a cathode of a lithium batteryand a method for fabricating the same, in which the above problems areeliminated or at least alleviated.

SUMMARY

In one embodiment, a cathode of a lithium battery includes a compositefilm. The composite film includes a carbon nanotube film structure and aplurality of active material particles dispersed therein.

Advantages and novel features of the present carbon-nanotube-basedcathode of the lithium battery and the related method for fabricatingthe same will become apparent from the following detailed description ofpreferred embodiments when taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present carbon-nanotube-based cathode of the lithiumbattery and the related method for fabricating the same can be betterunderstood with reference to the following drawings. The components inthe drawings are not necessarily to scale, the emphasis instead beingplaced upon clearly illustrating the principles of the presentcarbon-nanotube-based cathode of lithium battery and the related methodfor fabricating the same.

FIG. 1 is a schematic view of a cathode of a lithium battery, inaccordance with a present embodiment;

FIG. 2 is a schematic view of a composite film used in the cathode ofthe lithium battery of FIG. 1;

FIG. 3 is a flow chart of a method for fabricating the cathode of thelithium battery of FIG. 1; and

FIG. 4 shows a Scanning Electron Microscope (SEM) image of a carbonnanotube film structure, in accordance with the present embodiment.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate at least one preferred embodiment of the presentcarbon-nanotube-based cathode of the lithium battery and the relatedmethod for fabricating the same, in at least one form, and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made to the drawings to describe, in detail,embodiments of the present carbon-nanotube-based cathode of the lithiumbattery and the related method for fabricating the same.

Referring to FIG. 1 and FIG. 2, a cathode 10 in the present embodimentincludes a current collector 12 and a composite film 14 disposed on thecurrent collector 12. The current collector 12 can, beneficially, be ametal substrate. Quite suitably, the metal substrate is a copper sheet.The composite film 14 includes a carbon nanotube film structure 16 and aplurality of active material particles 18 dispersed in the carbonnanotube film structure 16.

The carbon nanotube film structure 16 includes at least two overlappedcarbon nanotube films. Each carbon nanotube film includes a plurality ofsuccessive carbon nanotube bundles joined end to end and are aligned inthe same direction. The at least two carbon nanotube films cross andoverlap with each other. The number of the carbon nanotube films and theangle between the aligned directions of the two adjacent carbon nanotubefilms is arbitrarily set.

In the present embodiment, a width of the carbon nanotube film structure16 can, suitably, be in the approximate range from 1 centimeter to 10centimeters, and a thickness of the carbon nanotube film structure 16can, usefully, be in the approximate range from 0.01 micron to 100microns. The carbon nanotube film structure 16 having a plurality ofmicropores defined by the spacing between adjacent carbon nanotubebundles. The diameter of the resulting micropores can, beneficially, beless than 100 nanometers.

Referring to FIG. 4, quite suitably, the carbon nanotube film structureincludes 200 carbon nanotube films overlapped with each other. In thepresent embodiment, the width of the carbon nanotube film structure 16is about 5 centimeters, and the thickness of the carbon nanotube filmstructure 16 is about 50 microns. The angle between the aligneddirections of two adjacent carbon nanotube films is about 90°. Thediameter of each of the micropores is about 60 nanometers.

The active material particles 18 are adsorbed to the wall of the carbonnanotubes by van der Waals attractive force or filled into the spacingbetween adjacent carbon nanotube bundles of the carbon nanotubestructure 16. The size of the active material particles 18 can,opportunely, be nano-scale. Quite suitably, the diameter of the activematerial particles 18 is in the approximate range from 3 nanometers to10 nanometers. In the present embodiment, the diameter of the activematerial particles 18 is about 6 nanometers.

The active material particles 18 can, opportunely, be made up oftransition metal oxides and mixed transition metal oxides such as spineltype lithium manganese oxide (e.g. LiMn₂O₄), olivine type lithium ironphosphate (e.g. LiFePO₄), and layered type lithium cobalt oxide (e.g.LiCoO₂).

It is to be understood that, the current collector 12 in the cathode 10of the lithium battery in the present embodiment is optional. In anotherembodiment, the cathode 10 of the lithium battery may only include thecarbon nanotube film structure 16. Due to a plurality of carbon nanotubefilms being piled to form a self-sustained and stable film structure,the carbon nanotube film structure 16 can be used as the cathode 10 inthe lithium battery without the current collector 12.

The carbon nanotube film structure 16 in the present embodiment hasextremely large specific surface area (i.e. surface area per gram ofsolid material). As such, a relatively large amount of active materialparticles 18 can be adsorbed to the walls of the carbon nanotubes orfilled into the micropores of the carbon nanotube structure 16.Accordingly, the charge/discharge capacity of the lithium battery usingthe above-described carbon-nanotube-based cathode can be improved.Further, because the active material particles are uniformly dispersedin the carbon nanotube film structure 16, the conductivity of thecathode can be enhanced.

Referring to FIG. 3, a method for fabricating the cathode 10 of thelithium battery includes the steps of: (a) providing an array of carbonnanotubes, quite suitably, providing a super-aligned array of carbonnanotubes; (b) pulling out at least two carbon nanotube films from thearray of carbon nanotubes, by using a tool (e.g., adhesive tape oranother tool allowing multiple carbon nanotubes to be gripped and pulledsimultaneously) to form a carbon nanotube film structure 16; and (c)providing a plurality of active material particles 18, dispersing theactive material particles 18 in the carbon nanotube structure 16 to forma composite film 14; (d) providing a current collector 12 and disposingthe composite film 14 on the current collector 12 to achieve the cathode10 of the lithium battery.

In step (a), a given super-aligned array of carbon nanotubes can beformed by the steps of: (a1) providing a substantially flat and smoothsubstrate; (a2) forming a catalyst layer on the substrate; (a3)annealing the substrate with the catalyst at a temperature in theapproximate range of 700° C. to 900° C. in air for about 30 to 90minutes; (a4) heating the substrate with the catalyst at a temperaturein the approximate range from 500° C. to 740° C. in a furnace with aprotective gas therein; and (a5) supplying a carbon source gas into thefurnace for about 5 to 30 minutes and growing a super-aligned array ofthe carbon nanotubes from the substrate.

In step (a1), the substrate can, beneficially, be a P-type siliconwafer, an N-type silicon wafer, or a silicon wafer with a film ofsilicon dioxide thereon. Quite suitably, a 4-inch P-type silicon waferis used as the substrate.

In step (a2), the catalyst can, advantageously, be made of iron (Fe),cobalt (Co), nickel (Ni), or any combination alloy thereof.

In step (a4), the protective gas can, beneficially, be made up of atleast one of nitrogen (N₂), ammonia (NH₃), and a noble gas. In step(a5), the carbon source gas can, advantageously, be a hydrocarbon gas,such as ethylene (C₂H₄), methane (CH₄), acetylene (C₂H₂), ethane (C₂H₆),or any combination thereof.

The super-aligned array of carbon nanotubes can, opportunely, be in aheight of about 200 to 400 microns and includes a plurality of carbonnanotubes parallel to each other and approximately perpendicular to thesubstrate. The super-aligned array of carbon nanotubes formed under theabove conditions is essentially free of impurities, such as carbonaceousor residual catalyst particles. The carbon nanotubes in thesuper-aligned array are packed together closely by van der Waalsattractive force. The carbon nanotubes in the super-aligned array ofcarbon nanotubes can, beneficially, be selected from single-walledcarbon nanotubes, double-walled carbon nanotubes, multi-walled carbonnanotubes, or any combination thereof.

It is to be understood that the method for providing the array of carbonnanotubes is not restricted by the above-mentioned steps, but any method(e.g. a laser vaporization method, or an arc discharge method) known inthe art.

In step (b), carbon nanotube films can, beneficially, be pulled out fromthe super-aligned array of carbon nanotubes by the substeps of: (b1)selecting a plurality of carbon nanotube segments having a predeterminedwidth; (b2) pulling the carbon nanotube segments at an even/uniformspeed to form at least two carbon nanotube films; (b3) overlapping theat least two carbon nanotube films to form a carbon nanotube filmstructure 16.

In step (b1), quite usefully, the carbon nanotube segments having apredetermined width can be selected by using a wide adhesive tape as thetool to contact the super-aligned array. In step (b2) the pullingdirection is, usefully, substantially perpendicular to the growingdirection of the super-aligned array of carbon nanotubes.

More specifically, during the pulling process, as the initial carbonnanotube segments are drawn out, other carbon nanotube segments are alsodrawn out end to end, due to the van der Waals attractive force betweenends of the adjacent segments. This process of drawing ensures asuccessive carbon nanotube film can be formed. The carbon nanotubes ofthe carbon nanotube film are all substantially parallel to the pullingdirection, and the carbon nanotube film produced in such manner is ableto formed to have a selectable, predetermined width.

In step (b3), after being pulled from the array of carbon nanotubes, thecarbon nanotube films can, usefully, be overlapped with each other toform a carbon nanotube film structure 16. It is noted that because thecarbon nanotubes in the super-aligned array in step (a) have a highpurity and a high specific surface area, the carbon nanotube film isadhesive. As such, adjacent carbon nanotube films are combined by vander Waals attractive force to form a stable carbon nanotube filmstructure 16. The number of carbon nanotube films and the angle betweenthe aligned directions of two adjacent carbon nanotube films may eitherbe arbitrarily set or set according to actual needs/use. Quite usefully,in the present embodiment, the carbon nanotube structure 16 includes 200carbon nanotube films, and the angle between the aligned directions oftwo adjacent carbon nanotube films can, opportunely, be about 90°.

Quite suitably, an additional step (e) of treating the carbon nanotubefilm structure 16 in the cathode 10 of the lithium battery with anorganic solvent can, advantageously, be further provided after step (b).

In step (e), the carbon nanotube film structure 16 can, beneficially, betreated by either of two methods: dropping an organic solvent from adropper to soak an entire surface of the carbon nanotube film structure16 or immersing the carbon nanotube film structure 16 into a containerhaving an organic solvent therein. The organic solvent is volatilizableand can be selected from the group consisting of ethanol, methanol,acetone, dichloroethane, chloroform, and combinations thereof. Quitesuitably, the organic solvent is ethanol. After being soaked by theorganic solvent, the carbon nanotube segments in the carbon nanotubefilm structure 16 can at least partially shrink into carbon nanotubebundles due to the surface tension created by the organic solvent. Dueto the decrease of the specific surface via bundling, the coefficient offriction of the carbon nanotube film structure 16 is reduced, but thecarbon nanotube film structure 16 maintains high mechanical strength andtoughness. As such, the carbon nanotube film structure 16 after thetreating process can be used conveniently. Further, after being treatedwith the organic solvent, due to the shrinking/compacting of the carbonnanotube segments into carbon nanotube bundles, the parallel carbonnanotube bundles in one layer are, relatively, distant (especiallycompared to the initial layout of the carbon nanotube segments) fromeach other and are oriented cross-wise with the parallel carbon nanotubebundles of adjacent layers. As such, the carbon nanotube film structure16 having a microporous structure can thus be formed (i.e., themicropores are defined by the spacing between adjacent bundles).

It is to be understood that the microporous structure is related to thelayer number of the carbon nanotube film structure 16. The greater thenumber of layers that are formed in the carbon nanotube film structure16, the greater will be the number of bundles in the carbon nanotubefilm structure 16. Accordingly, the spacing between adjacent bundles andthe diameter of the micropores will decrease.

It will be apparent to those having ordinary skill in the field of thepresent invention that the size of the carbon nanotube film structure 16is arbitrarily and depends on the actual needs of utilization (e.g. aminiature lithium battery). A laser beam can be used to cut the carbonnanotube film structure 16 into smaller size in open air.

In step (c), the composite film 14 can, advantageously, be formed by thesubsteps of: (c1) providing a preform or a precursor of the activematerial; (c2) immersing the carbon nanotube film structure 16 in thepreform or the precursor of the active material to form the compositefilm 14.

The active material can, opportunely, be transition metal oxides andmixed transition metal oxides such as spinel type lithium manganeseoxide (e.g. LiMn₂O₄), olivine type lithium iron phosphate (e.g.LiFePO₄), and layered type lithium cobalt oxide (e.g. LiCoO₂).

In one useful embodiment of step (c), the preform of the active materialcan, suitably, be a mixture of the active material and a solvent. Thesolvent can, beneficially, be selected from the group consisting ofwater, ethanol, acetone, and combinations thereof. Quite usefully, theactive material can be saturated in the solvent. The carbon nanotubefilm structure 16 can, advantageously, be immersed in the preform of theactive material for a period of time until the solvent volatilizedcompletely. Thus, the active material particles 18 can be uniformlydispersed in the carbon nanotube film structure 18.

In another embodiment of step (c), the preform can, usefully, be theactive material in gas state at elevated temperature. The carbonnanotube film structure 16 can be directly disposed in the gas of theactive material for 0.5 to 2 hours in a protective gas. After cooled toroom temperature, the active material particles 18 can be formed anduniformly dispersed in the carbon nanotube film structure 16. Theprotective gas can, beneficially, be made up of at least one of nitrogen(N₂), ammonia (NH₃), and a noble gas.

In step (c), the precursor is a mixture of at least two reactants forpreparing the active material. The reactants can, opportunely, be in gasstate, liquid state, or mixed with a solvent. The carbon nanotube filmstructure 16 can, suitably, be immersed in the precursor for a period oftime. After the reaction of the reactants, the active material particles18 can be formed and uniformly dispersed in the carbon nanotube filmstructure 16. It is to be understood that the impurities formed by thereaction can be eliminated by a washing/filtration step.

In the present embodiment, the preform is a saturated solution of LiCoO₂in water. The carbon nanotube film structure 16 is immersed in thepreform for several hours until the water dried up. Thus, in theresulted composite film 14, the LiCoO₂ particles are uniformly dispersedin the carbon nanotube film structure 16.

In step (d), the current collector 12 can, beneficially, be a metalsubstrate. Quite suitably, the metal substrate is a copper sheet.

It is to be understood that, in step (d), the current collector 12 inthe cathode of the lithium battery is optional. In another embodiment,the cathode of the lithium battery may only include the composite film14. Due to a plurality of carbon nanotube films being overlapped to forma self-sustained and stable film structure, the composite film 14 can besolely used as the cathode in the lithium battery without the currentcollector 12.

The cathode 10 of the present embodiment includes the carbon nanotubefilm structure 16 and the active material particles 18 uniformlydispersed therein. As such, the conductivity of the cathode can beenhanced. Further, the capacity of the lithium battery using theabove-described cathode 10 can be improved due to the reduced resistancethereof. Additionally, the method for fabricating the above-describedcathode 10 is simple and suitable for mass production.

Finally, it is to be understood that the above-described embodiments areintended to illustrate rather than limit the invention. Variations maybe made to the embodiments without departing from the spirit of theinvention as claimed. The above-described embodiments illustrate thescope of the invention but do not restrict the scope of the invention.

1. A method for fabricating a cathode of a lithium battery, the methodcomprises the steps of: (a) providing an array of carbon nanotubes; (b)pulling out, by using a tool, at least two carbon nanotube films fromthe array of carbon nanotubes; (d) overlapping the at least two carbonnanotube films along different direction to form a carbon nanotube filmstructure where the carbon nanotubes are crossed with each other; and(c) providing a preform or a precursor of the cathode active material,and immersing the carbon nanotube film structure in the preform or theprecursor of the cathode active material to form the composite film, andthereby, achieving the cathode of the lithium battery.
 2. The method asclaimed in claim 1, wherein the step (b) further comprises the substepsof: (b1) selecting a plurality of carbon nanotube segments having apredetermined width from the array of carbon nanotubes; and (b2) pullingthe carbon nanotube segments at a uniform speed and perpendicular to thegrowing direction of the array of the carbon nanotubes, to form the atleast two carbon nanotube films.
 3. The method as claimed in claim 1,wherein a step of treating the carbon nanotube film structure with anorganic solvent is further provided after step (b), and the organicsolvent comprises at least one material selected from the groupconsisting of ethanol, methanol, acetone, dichloroethane, chloroform,and combinations thereof.
 4. The method as claimed in claim 1, whereinthe preform of the cathode active material is a mixture of the cathodeactive material and a solvent, or the cathode active material is in agaseous state.
 5. The method as claimed in claim 4, wherein the cathodeactive material is saturated in the solvent.
 6. The method as claimed inclaim 4, wherein the cathode active material in the gaseous state iscooled in protective gas to form a plurality of cathode active materialparticles, and the protective gas comprises at least one gas selectedfrom the group consisting of nitrogen (N₂), ammonia (NH₃), and a noblegas.
 7. The method as claimed in claim 1 further comprising a step ofcutting the carbon nanotube film structure into a predetermined shapeand size.
 8. The method as claimed in claim 1, wherein a step (d) ofproviding a current collector and disposing the composite film on thecurrent collector is further provided after step (c).
 9. The method asclaimed in claim 1, wherein the preform is a solution having LiCoO₂particles uniformly dispersed therein.
 10. The method as claimed inclaim 1, wherein in step (b), an angle between aligned directions of theat least two carbon nanotube films is about 90°.
 11. The method asclaimed in claim 7, wherein the step of cutting the carbon nanotube filmstructure comprises using a laser beam.
 12. The method as claimed inclaim 3, wherein the step of treating the carbon nanotube film structurewith the organic solvent comprises dropping the organic solvent to soakan entire surface of the carbon nanotube film structure.
 13. The methodas claimed in claim 3, wherein the step of treating the carbon nanotubefilm structure with the organic solvent comprises: immersing the carbonnanotube film structure into a container having the organic solventtherein.
 14. The method as claimed in claim 1, wherein the carbonnanotube film structure comprises a plurality of micropores, the cathodeactive material particles are absorbed on walls of the carbon nanotubesor filled into the plurality of micropores.
 15. The method as claimed inclaim 1, wherein the composite film comprises a self-sustainedstructure.
 16. The method as claimed in claim 1, further comprising astep of drying the composite film after the step (c).